DACITE BLOCK AND ASH AVALANCHE HAZARDS IN MOUNTAINOUS TERRAIN: 2360 YR. BP ERUPTION OF MOUNT MEAGER, BRITISH COLUMBIA by MARTIN L. STEWART B.Sc, (Honours), Carleton University, 1998 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF EARTH AND OCEAN SCIENCES We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA December 2002 © Martin L. Stewart, 2002 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of LoM^ r QatA^ Sc/t^n? The University of British Columbia Vancouver, Canada Date IB * zooi DE-6 (2/88) Abstract The Mount Meager volcanic complex hosts deposits from the youngest known explosive volcanic eruption in Canada (2360 yr. BP). These deposits reflect the consequences of erupting dacite magmas into a region of extreme topographic relief. Regions of this kind represent one of the most hazardous and, potentially, high risk natural environments on the planet. Mapping of the Pebble Creek Formation deposits has elucidated a unique distribution of hazardous events of varying intensity, timing, and frequency associated with the 2360 yr. BP eruption event. For example, lavas erupted onto the steep slopes of the volcano failed under gravitational stresses producing hot block and ash avalanches. During the later stages of this extrusive activity, cold rock avalanches were produced on the oversteepened slopes of the volcano, in one case mixing with a hot block and ash avalanche. Both volcanic and rock avalanches were highly mobile, travelling down the slopes of the volcano, fdling the valley and running up the opposite valley wall. Deposits from these events preserve features that are diagnostic of their hot volcanic or cold mass wasting origins and allow these two deposit types to be distinguished in the field. Discrimination is essential because these deposits are superficially very similar in appearance and may be complexly interlayered, but they have different hazard implications. The steep narrow valleys surrounding the Mount Meager volcano provide an efficient catchment for volcanic and rock avalanches. During the 2360 yr. BP eruption cycle thick block and ash avalanche deposits formed a natural dam against the flow of the Lillooet River, a major drainage system. Volume estimates and flow rates of the current Lillooet River suggests there was an ongoing competition between building of the ii pyroclastic dam and filling of the lake. Gradual buildup of the dam was periodically interrupted by overtopping and catastrophic failure of the dam by the encroaching lake. This catastrophic failure released high volumes of water, flooding the Lillooet valley as far as Pemberton, B.C. and produced highly destructive debris flows. Properly assessing the distribution and nature of volcanic and associated hazards in mountainous regions features is requisite for formulating a risk model for areas such as Mount Meager. Mapping of the deposits elucidates the distribution, and frequency of past hazardous events. The detailed stratigraphic analysis presented in this study demonstrates the destructive potential of these events based on the intensity and time of onset for analogous modern events. iii Table of Contents Abstract 11 Table of Contents iv List of Figures vi List of Tables viii Insert ix Acknowledgments x 1. Introduction 1 2. The Pebble Creek Formation 2.1 Introduction 5 2.2 Geologic Setting 5 2.3 Volcanic Deposits 11 2.4 Non-volcanic Deposits 21 2.5 Eruption History 26 3. Discriminating Hot Versus Cold Avalanches 3.1 Introduction 30 3.2 Criteria for Distinguishing Deposits in the Field 31 3.3 Comparing Rock and Volcanic Avalanche Hazards 35 4. Block and Ash Avalanche Formation 4.1 Introduction 39 4.2 Duration of Block and Ash Avalanche Production 39 4.3 Source of Block and Ash Avalanches 42 iv 4.4 Failure Mechanisms of Viscous Lavas 44 4.5 Change in Behaviour of Erupting Lavas 56 5. Failure of a Pyroclastic Dam 5.1 Introduction 61 5.2 Pyroclastic dam structure 62 5.3 Filling of the paleo-Salal Lake 64 5.4 Dam Failure and Flooding 67 6. Discussion: Volcanic Hazards of a Dacite Erupting in Mountainous Terrain 72 List of References 79 Appendices I. Geochemical Data 93 II. Clast Count Data 100 III. Volume Maps and Cross-Section Key 102 IV. Lake Volume Data 108 v List of Figures 1. Distribution of Quaternary volcanoes in the Garibaldi volcanic belt, including rock avalanches associated with the major edifices (map) 6 2. Distribution of deposits within the Lillooet River valley (sketch map based on outcrop map insert) 9 3. Chemical discrimination plot of major elements from whole rock and electron microprobe analysis (from LeBas et al., 1986) 10 4. Images of pyroclastic fallout and pumiceous pyroclastic flow deposits 12 5. Stratigraphic sections of the Pebble Creek formation volcanic deposits 13 6. Clast abundances in pyroclastic fall section and images of a welded pumice breccia accessory clast 14 7. Overview image of the pyroclastic deposits from the Northern slopes of the Lillooet valley 17 8. Images of block and ash avalanche deposits and lava flow 18 9. Images of rock avalanche, reworked pumice, delta and outburst flood deposits 22 10. Stratigraphic sections of non-volcanic deposits illustrating lake stand elevations on lateral continuity of the late rock avalanche deposit 24 11. Flow chart for discriminating rock avalanches and volcanic avalanches from field criteria 32 12. Images clast features that illustrate hot versus cold origins for avalanche deposits ..33 13. Cross sections of the angle of reach and runout for rock avalanches and block and ash avalanches 45 14. Schematic illustrations of simple lava flow models including: vi a) a static block model lava flow for calculating critical resolved shear stresses 47 b) a steady state lava flow velocity profde to calculate strain deriving from a velocity gradient 47 15. Temperature dependent viscosity and relaxation timescale curves: a) temperature dependent viscosities of some natural volcanic lavas ....50 b) relaxation timescales based on (a) and relationships in Dingwell and Webb (1990) 50 16. Curves comparing critical resolved shear stresses acting on the base of a lava flow and the glass transition calculated from rheological properties of a Newtonian viscous liquid 52 17. Slope values of the North face of Mount Meager along the axis of travel of mapped avalanches 59 18. Idealized cross section and images of the deposits comprising the pyroclastic dam that blocked the Lillooet River 63 19. Image and cross sections of the pyroclastic dam and canyon eroded into the dam ...65 20. Lake highstand and peak discharge curves a) lake elevations based on filling rates and relevant stratigraphic features 68 b) peak water discharge curve for dam failure based on the lake volume and dam height 68 21. Conceptual curve of dam elevation and lake elevation versus time 69 22. Time scales of eruption processes and associated hazards 74 23. Summary chart of the characteristics of hazards affecting the Lillooet valley 78 vii List of Tables 1. Summary of recorded avalanche and eruption events in the Mount Meager volcanic complex over the last 2 million years 7 2. Criteria from field observations allowing discrimination of hot (volcanic) and cold (mass wasting) avalanche deposits 35 3. Summary of extrusion rates recorded from historical eruptions of silicic volcanoes 41 4. Example of parameters and solution for length scales of strain in a steady state velocity profile which would result in the failure of a Newtonian viscous liquid 54 viii Insert DATA CD: Files Include: 1. Data - Geochemical data files (.xls format) 2. MeagerGIS - GIS mapping data files (ARCView format) 3. PCFmap - Digital outcrop map and field station map (.pdf format) 4. Thesis - Thesis (.pdf format) 5. Thesiscdr - Draft: thesis figures (xdr format) 6. Thesisdoc - Draft: thesis text (.doc format) ix Acknowledgments I would like to thank my advisor, Kelly Russell for his patience and vigilance during the course of this thesis. Contributions from committee members including Cathie Hickson, Greg Dipple, Mati Raudsepp proved invaluable. Financial support for this research included a University Graduate Fellowship (1999), Mineralogical Association of Canada National Scholarship (2000), NSERC PGS B Scholarship (2001), Lorntzen Scholarships (1999-2001), and NSERC operating grant #589820 (J.K. Russell). Field and technical support was provided in part by the Geological Survey of Canada and Garth Carefoot (Great Pacific Pumice Inc.). I am indebted to Mark Stasiuk, Kirstie Simpson, Kaz Shimamura, Oldrich Hungr, John Clague and members of the University of British Columbia (UBC) faculty for enlightened discussion and assistance on the issues presented here. Steve Quane deserves special mention for helping me survive my time at UBC and in Vancouver. A debt of gratitude goes to Scotty, Steve, Justin and my grad student friends for always keeping it real. To all the undergraduates at UBC I have had the privilege to become acquainted with, my family from 7th avenue, and all the field assistants over the years, thanks for the good times. Finally thanks to my family back home, here's another one for the shelf. x 1. Introduction Chapter 1 Introduction The margins of convergent plates commonly have landscapes that feature large volcanic edifices (in particular, stratovolcanoes) situated in regions of extreme topographic relief (Wood and Kienle, 1990; Perfit and Davidson, 2000).